Task Migration and its issues


Task Migration
         It is the movement of an executing task from one host processor in distributed computing system to another.
         Task placement is the selection of a host for a new task and the creation of the task on that host.

Benefits of task Migration
         Load Balancing: -Improve performance for a distributed computing system overall or a distributed application by spreading thee load more evenly over a set of host.
         Reduction in Communication overhead: - by locating on one host a group of tasks with intensive communication amongst them.
         Resource access:-not all resources are available across the network, a task may need to migrate in order to access a special device, or to satisfy a need for a large amount of physical memory
         Fault tolerance:-allowing long running processes to survive the planned shutdown or failure of a host.

Steps involved in task migration
         Suspending the task on the source.
         Extracting and transmitting the state of the task to destination.
         Reconstructing the state on the destination
         Deleting the task on the source and resuming the task’s execution on the destination.

Task Migration Issues
There are three issues in the task migration:-
         State Transfer
         Location transparency
         Structure of a migration mechanism

State Transfer
         The cost to support remote execution.
         Freezing the task.
         Obtaining and transferring the state.
         Unfreezing the task.

         Residual dependencies:-Refer to the amount of resources a host of a migrated task continues to dedicate to service requests from the migrated task. They are undesirable for three reasons:-Reliability, performance and complexity.

State Transfer mechanisms
         Recopying the state:-bulk of the task is copied to the new host before freezing the task.
         Location transparent file access mechanism
         Copy on reference: - just copy what is migrated task need for its execution.

Location Transparency
         Task Migration should hide the locations of tasks. Location transparency in principle requires that name (process name, file name) be independent of their locations (host names).
         Uniform name spacer throughout the system.

Structure of a migration mechanism
         Typically. Here will be interaction between the task migration mechanism, the memory management system, the interprocess communication mechanism and the file system.
         The mechanism can be designed to be independent for one another so that if one mechanism’s protocol changes; the other’s need not the migration mechanism can be turned off without interfering with other mechanisms.

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